The Free T3 Test: The free T3 test measures triiodothyronine (T3) that is available to react with the body’s cells. In its free form, FT3 is cleaved from carrier protein molecules and has the ability to react with cells, causing the actions associated with triiodothyronine. While the body produces very little T3, the majority of T3 is formed from the conversion (de-iodination) of T4. In its available form, FT3 is about 7-10 times more potent than FT4. In Graves’ disease, many patients have T3 thyrotoxicosis, a condition of increased T3 production and conversion from T4. Patients who are monitored with FT4 alone can present an incomplete clinical picture. FT3 is increased in Graves’ disease, T3 thyrotoxicosis, TSH dependent hyperthyroidism, and from medications increasing T4 to T3 conversion, such as acetaminophen. FT3 is decreased in nonthyroidal illness (low T3 syndrome), hypothyroidism, and by poor T4 conversion syndromes caused by metabolic changes or certain medications. The usual reference range for FT3 is 230-420 pg/dl.
NOTE: Methods for free thyroid hormone can rarely be falsely elevated in patients who have abnormally high or low levels of thyroxine-binding-globulin (TBG) caused by conditions of dysalbuminemia, or very high levels of estrogen or non-steroidal anti-inflammatory medications. Although these interferences are rarely seen in free thyroid hormone tests, methods for measuring free thyroid hormone using dialysis methods, which eliminate all protein related interferences, are now available. Their use is generally not recommended because they are time-consuming and more expensive. Total T4 and T3 levels are always affected by these substances and are no longer recommended by monitoring thyroid function.
The TSH Test: TSH refers to the pituitary hormone, thyroid stimulating hormone, which is also known as thyrotropin. TSH is normally secreted at a constant pulsating rate, which peaks at night. TSH stimulates thyroid cells, ordering them to produce and release thyroid hormone. When the pituitary, via a signal from the hypothalamus, recognizes that the body has too much thyroid hormone for its needs, it slows down or stops producing TSH. Before thyroid hormone levels become abnormally high, TSH is low and is often suppressed to levels <.01 mu/ml. A low TSH does not signify that thyroid hormone levels are excessively high.
Go To Page: 1 2
